U.S. patent application number 11/005785 was filed with the patent office on 2006-06-08 for controlled densification of fusible powders in laser sintering.
This patent application is currently assigned to 3D Systems, Inc.. Invention is credited to Khalil M. Moussa, Stephen A. Ruatta.
Application Number | 20060119012 11/005785 |
Document ID | / |
Family ID | 35516984 |
Filed Date | 2006-06-08 |
United States Patent
Application |
20060119012 |
Kind Code |
A1 |
Ruatta; Stephen A. ; et
al. |
June 8, 2006 |
Controlled densification of fusible powders in laser sintering
Abstract
The invention relates to a method producing parts using laser
sintering wherein a fusible powder is exposed to a plurality of
laser scans at controlled energy levels and for time periods to
melt and densify the powder and in the substantial absence of
particle bonding outside the fusion boundary. Strength is improved
up to 100% compared to previous methods. An example includes a
relatively high energy initial scan to melt the powder followed by
lower energy scans controlled to densify the melt and separated in
time to dissipate heat to the surrounding part cake. The rate and
extent to which the powder particles are fused together can be
controlled so that each successive scan can be used to fuse the
particles together in discreet incremental steps. As a result, the
final dimensions of the part and its density and mechanical
properties can be improved compared to conventional methods and
part growth avoided.
Inventors: |
Ruatta; Stephen A.; (South
Pasadena, CA) ; Moussa; Khalil M.; (Stevenson Ranch,
CA) |
Correspondence
Address: |
3D SYSTEMS, INC.
26081 AVENUE HALL
VALENCIA
CA
91355
US
|
Assignee: |
3D Systems, Inc.
Valencia
CA
|
Family ID: |
35516984 |
Appl. No.: |
11/005785 |
Filed: |
December 7, 2004 |
Current U.S.
Class: |
264/497 ;
425/174.4 |
Current CPC
Class: |
C23C 24/10 20130101;
B29C 64/153 20170801; B33Y 10/00 20141201; C23C 26/02 20130101;
B33Y 70/00 20141201 |
Class at
Publication: |
264/497 ;
425/174.4 |
International
Class: |
B29C 35/08 20060101
B29C035/08; B29C 41/02 20060101 B29C041/02 |
Claims
1. A method of laser sintering comprising the steps of: a.
providing a layer of fusible powder; b. exposing a predetermined
target area of the powder within the layer to a plurality of scans
of laser energy at a controlled energy level and along the same
fusion path for a time to maintain the powder in said target area
at or above its melting point in the absence of fusing adjacent
powder in the layer outside the target area; and c. repeating the
steps of providing and exposing a plurality of times to produce a
three dimensional object, the plurality of scans being
characterized by an initial scan having an energy level that is
higher than subsequent scans.
2. The method according to claim 1, wherein the step of exposing
the target area to a plurality of scans comprises two or more
scans.
3. The method according to claim 1, wherein within the step of
repeating the steps of providing and exposing, the target area
varies among repetitions.
4. A method according to claim 1, wherein the step of providing a
layer of fusible powder comprises depositing a layer of powder of
predetermined thickness onto a part cake surface wherein said
powder is selected from group consisting of nylon-11, nylon-12,
polystyrene, polybutylene terephthalate, and polyacetal.
5. A method according to claim 1, wherein the step of exposing the
fusible powder to a plurality of scans produces a fused mass of
predetermined geometry.
6. A method according to claim 1, wherein the step of exposing the
fusible powder to a plurality of scans comprises exposing the
powder to subsequent scans each having successively lower energy
levels.
7. A method according to claim 1, wherein the step of exposing the
target area of fusible powder to a plurality of scans comprises:
scanning said target area of fusible powder a first time with laser
energy to melt said powder; allowing heat to dissipate to powder
beyond said target area while maintaining the target area in a
melted state; and rescanning said layer of powder a second time
with laser energy sufficient and for a time sufficient to maintain
the target area in a melted condition.
8. A method according to claim 7, wherein the first scan has an
energy level that is higher than energy level of any subsequent
scan.
9. A method according to claim 1, wherein the step of exposing the
fusible powder to a plurality of scans of laser energy further
comprises supplying a CO.sub.2 laser.
10. A method according to claim 1, wherein the step of exposing
said fusible powder to a plurality of scans of laser energy further
comprises scanning the layer in vector fashion.
11. A method of producing a part comprising the steps of:
depositing a layer of fusible powder onto and adjacent to a
preselected target area; scanning a directed energy beam over the
target area in an initial scan to melt the powder in the target
area; allowing heat to dissipate into adjacent powder while
maintaining the target area powder melted and in the absence of
fusing adjacent powder; rescanning the target area at least one
time along the same fusion path at a lower energy level than the
initial scan to maintain the target area powder in a melted state
and in the absence of fusing adjacent powder; repeating the above
steps to form a three dimensional part.
12. The method according to claim 11, comprising the further steps
of: providing a controller operatively connected to the directed
energy beam; and supplying the controller with the boundaries of
each cross-sectional region of the part.
13. The method according to claim 12, comprising the further steps
of: providing a computer; and supplying the overall dimensions of
the part to the computer, the computer determining the boundaries
of each cross-sectional region of the part.
14. The method according to claim 12, wherein the scanning steps
include the steps of moving the aim of the beam in a raster
scan.
15. The method according to claim 11, wherein the rescanning step
includes exposing the target area powder to subsequent scans each
having successively lower energy levels.
16. Apparatus for laser sintering in layer-wise fashion fusible
powders comprising a computer control system for providing multiple
laser scans along the same fusion path at different energy level
intensities in each layer of powder, each successive laser scan
being at a lower energy level intensity than a previous scan, the
multiple laser scans in multiple layers forming a
three-dimensionsal object.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates generally to a method of rapid
prototyping and manufacturing and, more particularly, to laser
sintering.
[0002] Rapid prototyping and manufacturing (RP&M) is the name
given to a field of technologies that can be used to form
three-dimensional objects rapidly and automatically from computer
data representing the objects. In general, rapid prototyping and
manufacturing techniques build three-dimensional objects,
layer-by-layer, from a working medium utilizing sliced data sets
representing cross-sections of the object to be formed. Typically
an object representation is initially provided by a Computer Aided
Design (CAD) system. RP&M techniques are sometimes referred to
as solid imaging and include stereolithography, ink jet printing as
applied to solid imaging. and laser sintering, to which the
invention is directed.
[0003] Laser sintering apparatus dispenses a thin layer of fusible
powder, often a fusible polymer powder or polymer coated metal,
over a bed of the powder and then applies thermal energy to melt
those portions of the powder layer corresponding to a cross-section
of the article being built in that powder layer. Lasers typically
supply the thermal energy through modulation and precise
directional control to a targeted area of the powder layer.
Conventional selective laser sintering systems, such as the
Vanguard system available from 3D Systems, Inc., use carbon dioxide
lasers and position the laser beam by way of galvanometer-driven
mirrors that deflect the laser beam. The apparatus then dispenses
an additional layer of powder onto the previously fused layer and
repeats the process of melting and selective fusing of the powder
in this next layer, with fused portions of later layers fusing to
fused portions of previous layers as appropriate for the article,
until the article is complete. These articles are sometimes
referred to as "built parts."
[0004] A computer operates the control system for the laser,
programmed with information indicative of the desired boundaries of
a plurality of cross sections of the part to be produced. The laser
may be scanned across the powder in raster fashion or vector
fashion. In vector fashion, the laser beam traces the outline and
interior of each cross-sectional region of the desired part. In a
raster scan, a modulated laser beam scans a repetitive pattern
across the powder. In some applications, cross-sections of articles
are formed in a powder layer by fusing powder along the outline of
the cross-section in vector fashion, either before or after a
raster scan that "fills" the area within the vector-drawn
outline.
[0005] Detailed descriptions of laser sintering technology may be
found in U.S. Pat. Nos. 4,863,538; 5,132,143; and 4,944,817, all
assigned to Board of Regents, The University of Texas System, and
in U.S. Pat. No. 4,247,508 to Housholder.
[0006] Laser sintering technology enables the direct manufacture of
three-dimensional articles of high resolution and dimensional
accuracy from a variety of fusible materials, including
polystyrene, some nylons, other plastics, and composite materials,
including polymer coated metals and ceramics. Laser sintering may
be used for the direct fabrication of molds from a CAD database
representation of the object to be molded. Computer operations
"invert" the CAD database representation of the object to be formed
to directly form the negative molds from the powder.
[0007] Laser sintering depends upon thermal control of the process
in the part cake to obtain good three-dimensional parts. The
sources of thermal energy are the laser, cylinder heaters that
preheat the powder in powder feed cylinders that supply a powder
layer to the apparatus, radiant heaters to heat the powder prior to
deposit on the laser target area, the radiant heater for the laser
target area, and the laser. The laser is typically a CO.sub.2 laser
that scans the fresh powder layer to fuse the powder particles in
the desired areas.
[0008] The increasing number of applications for laser sintered
products has resulted in a demand for built parts having improved
physical properties. Present commercial systems effectively deliver
powder and thermal energy in a precise and efficient way.
Nevertheless, laser sintered parts are sometimes dimensionally
distorted and may not have the strength of, for example, injection
molded plastic parts.
[0009] The sintering process may leave void spaces between the
individual particles that reduce the strength of the built part.
Increasing the thermal energy supplied to the fusible powders can
result in dimensionally distorted parts. Heated particles at the
boundaries of the target area may melt and adhere to particles
immediately outside the targeted area. The interior of individual
powder particles may become melted causing excess material to flow
into void spaces that exist between the surrounding particles. One
or more layers may experience an overall increase in dimensions
from the nominal values calculated by the CAD program. The
undesirable increase is commonly referred to as "growth" and
reflects that the mean value of the dimensions obtained varies an
unacceptable degree from the calculated nominal value. Such growth
may make a sintered part unusable for its intended purpose.
[0010] Thus, there exists a need for a method of using laser
sintering to produce parts that are accurate and have high
strength.
BRIEF SUMMARY OF THE INVENTION
[0011] The invention provides an improved method of forming a three
dimensional object using laser sintering and an apparatus for
accomplishing the method. Each powder layer is scanned a plurality
of times with a laser along the same fusion path without
significant cooling between the scans so that the powder in the
fusion path remains at or near the melting point without extending
outside the fusion boundary and without fusing powder outside the
part boundary. Density and tensile strength, tensile modulus, and
elongation-to-break are all controlled more effectively and in the
absence of significant growth comparable to injection molded parts.
While not wishing to be bound by theory, it is believed that
controlling the laser intensity or energy level and frequency of
scanning according to the invention dissipates the heat of fusion
into the surrounding part cake without causing the melted powder to
extend beyond the desired fusion boundary and without resulting in
fusion of the powder outside the fusion boundary.
[0012] Exposing each layer to multiple laser scans allows the rate
at which the individual particles are fused to be controlled so
that the molten material flows together in discreet incremental
steps. Each successive scan provides that amount of energy needed
to keep the powder at or near its melting point, and normally
slightly above its melting point. The softened particles can then
flow together filling void spaces in the fusion path to produce a
solidified mass. Growth is avoided, accuracy is improved, and the
density of the parts can be increased to that comparable with
injection molding.
[0013] The power of each successive scan can be varied to reduce
the time taken to apply multiple laser scans along the same laser
path and to achieve the desired part density and dimensions. A
relatively higher energy level initial scan should be sufficient to
raise the powder to the region of its melting point and to melt the
outer regions of the powder particles in the absence of excessive
melting that could result in unwanted growth. Successive scans
should be applied at a lower energy level sufficient to maintain
the powder at or near and normally slightly above its melting point
and at the desired low viscosity since less heat is required in the
subsequent scans to maintain or increase the temperature of the
powder and thereby reduce the viscosity of the melt. Successive
scans can be at successively lower energy levels. As a result, the
amount of time required for multiple scans along the same path can
be reduced.
[0014] Thus, the invention provides a method whereby parts can be
produced with laser sintering that have improved dimensional
accuracy and mechanical properties by multiple laser scans along
the same fusion path. The invention also includes a laser sintering
apparatus that provides for multiple scans along the same fusion
path at different laser beam intensities or energy levels.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0015] Having thus described the invention in general terms,
reference will now be made to the accompanying drawings, which are
not necessarily drawn to scale, and wherein:
[0016] FIG. 1 is a front perspective cutaway view of a conventional
laser sintering system illustrating the internal structure of a
laser sintering apparatus of the prior art;
[0017] FIG. 2 is a schematic illustration of the principal
operating systems of the laser sintering apparatus of FIG. 1;
[0018] FIG. 3A is a graphical illustration depicting the amount of
heat introduced during a typical high energy laser scan of the
prior art to melt a fusible powder to form a three-dimensional
object;
[0019] FIG. 3B is a graphical illustration of the amount of heat
introduced in the practice of the invention by three successive
laser scans at a reduced energy as compared to FIG. 3A that is
sufficient to maintain the powder at or slightly above its melting
point;
[0020] FIG. 3C is a graphical illustration of the amount of heat
introduced in an alternative embodiment of the practice of the
invention by four successive laser scans, the first of which is at
somewhat higher energy designed to bring a fusible the powder up to
melting and the subsequent three of which are at a reduced energy
sufficient to maintain the powder at or slightly above its melting
point; and
[0021] FIG. 4 is a graphical illustration of the relationship
between density of the built part made in accordance with the
invention and the time for which the built part is maintained at an
elevated temperature at or near its melting point.
DETAILED DESCRIPTION OF THE INVENTION
[0022] The invention now will be described more fully hereinafter
with reference to the accompanying drawings, in which some, but not
all embodiments of the invention are shown. Indeed, the invention
may be embodied in many different forms and should not be construed
as limited to the embodiments set forth herein; rather, these
embodiments are provided so that this disclosure will satisfy
applicable legal requirements. Like numbers refer to like elements
throughout.
[0023] A conventional selective laser sintering system having a
dual feed for fusible powder of the type currently sold by 3D
Systems, Inc. of Valencia, Calif. is illustrated in FIGS. 1 and 2.
Conventional sintering systems typically include a directed energy
beam and a controller for aiming the energy beam. As shown in the
Figures, the directed energy beam is a carbon dioxide laser 108.
Its scanning system 114 is depicted mounted in unit 100 above a
process chamber 102 that includes a part bed or part cake 132 in
which the powder is fused to build a part. The process chamber
maintains the appropriate temperature and atmospheric composition
for the fabrication of the part. The atmosphere is typically an
inert atmosphere, such as nitrogen. It is also possible to operate
the process chamber under vacuum.
[0024] The part cake includes regions of fused and unfused powder
and contains the part that is being built. The region of fused
powder is the built part 131. The region of unfused powder is
designated as 106 in FIG. 2. The system as illustrated is a dual
feed system that includes two powder cartridges 124, 126 which
alternately supply fresh layers of powder to the part bed from
opposite directions. A leveling device, which may be a
counter-rotating roller 130, or a knife including a doctor blade or
similar apparatus, distributes the fresh powder over the powder
bed. Typically, a first portion of the powder is deposited within
the target area followed by scanning the powder with the laser
beam, and repeating with a second portion of powder and scanning.
These steps are repeated as necessary, layer-by-layer, to produce
the desired object.
[0025] Laser 108 generates a laser beam 104 that scanning system
114 aims at a target surface 110 in part cake 132. Scanning system
114 includes galvanometer-driven mirrors that deflect the laser
beam. The laser and galvanometer systems are isolated from the hot
process chamber 102 by a laser window 116, as seen in FIG. 2. The
deflection and focal length of the laser beam is controlled, in
combination with the modulation of laser 108, to direct a
preselected laser energy to those locations of the fusible powder
layer corresponding to the cross-section of the article to be
formed in that layer, which is target surface 110. Scanning system
114 scans the laser beam across the powder in a vector fashion with
the laser beam intersecting the powder surface in the target area
110. It should be recognized that a scanning system capable of a
raster scan could also be used.
[0026] The scanning system typically includes a controller (not
shown) that may be operatively connected to the directed energy
beam and the galvanometers. The controller controls the position
and direction of the energy beam through the galvanometers and
modulates the energy level of the beam. Typically, the controller
comprises a computer or equivalent device that can be used to
control the operation of the system. The computer may include a
CAD/CAM system, as where the computer is given the overall
dimensions and configuration of the part to be made and the
computer determines the boundaries of each cross-sectional region
of the part. Using the preselected nominal boundaries, the computer
controls fusing of each layer corresponding to the cross-sectional
slices taken through the part. In an alternative embodiment, the
computer is simply programmed with the boundaries of each
cross-sectional region of the part.
[0027] As shown in FIG. 2, two powder cartridges 124, 126 feed
powder into the process chamber by means of push-up pistons 125,
127, respectively. Piston 127 first pushes up a measured amount of
powder 123 from powder feed cartridge 126 and a counter-rotating
roller 130 pushes and spreads the powder over the part cake 132 in
a uniform manner. The counter-rotating roller 130 passes completely
over part cake 132, including the target area 110, and powder
cartridge 124 to produce a level layer of powder extending from one
side of the process chamber to the other. Residual powder, if any,
is deposited into an overflow receptacle 136. The system then
directs the laser beam to fuse powder in the target area. In an
analogous manner, powder cartridge 124 supplies the next layer of
powder to the roller via piston 125, which is spread over the
powder bed and powder cartridge 126 with excess deposited into
overflow receptacle 138. The dual feed system operates in this
manner until all the layers are deposited and fused and the part is
built. Piston 128 lowers the part cake after each layer is fused so
as to accommodate the next layer of fresh powder.
[0028] Another powder delivery system uses overhead hoppers to feed
powder from above and either side of target area 110 in front of a
delivery apparatus such as a wiper or scraper. It should be
recognized that the invention can be used with a wide variety of
different systems and is not limited to any particular laser
sintering system or design.
[0029] Positioned near the top of the process chamber 102 above
each powder chamber and above the part bed 132 are radiant heater
elements. Heater elements 122 pre-heat the feed powder and maintain
the feed powder in a heated condition for deposit on the powder bed
and to minimize thermal shock. Heater element 120 heats the part
cake and desirably maintains the target area at a temperature just
below melting. Laser window 116 is situated above and on an axis
that positions it within radiant heater elements 120, which define
a central opening for the laser window. These heater elements 120
may be ring shaped panels or radiant heater rods that surround the
laser window 116. The rings may be rectangular or circular.
[0030] A wide variety of fusible media may be used in the practice
of the invention. Suitable powders include amorphous, crystalline,
and semi-crystalline powders, although not necessarily with
equivalent results. Semi-crystalline powders melt and recrystallize
well in the practice of the invention whereas crystalline powders
can exhibit brittleness and amorphous powders can tend to produce
built parts exhibiting growth. Growth in amorphous parts is in
distinction to amorphous parts produced by injection molding where
the polymer melt is confined by the mold surfaces. Crystalline
powders tend to have melting points that equal or are very close to
the temperature of recrystallization, which is not desirable in the
practice of laser sintering. Semi-crystalline powders having a
melting point well distinct from their recrystallization point work
well in the practice of the invention.
[0031] Suitable powders include semi-crystalline polymers, such as,
nylons, polystyrenes, polybutylene terephthalate (PBT), ethylenes,
propylenes, and polyacetals (PA), and copolymers and homopolymers
thereof. A particularly useful fusable media is nylon-12 which is
available from 3D Systems, Inc. under the trademark
DuraForm.RTM..
[0032] In accordance with the invention, exposing each powder layer
to a plurality of laser scans at various preselected energies
improves the mechanical properties and dimensional accuracy of the
resulting part. Scanning each layer at an energy level to maintain
the powder at or slightly above its melting point helps to control
the amount of heat that is applied to the powder so that the amount
of melting that each particle undergoes can be limited. As a
result, the decrease in viscosity of the powder can be controlled
and growth can be reduced or substantially eliminated.
[0033] Previously scanning at low energies was considered somewhat
undesirable because of the resultant decrease in viscosity of the
targeted particles that results could be sufficient to reduce or
preclude the particles and successive layers from effectively
fusing. As a result, the mechanical stability of the part could be
adversely impacted. In sharp contrast, however, it has now been
determined that exposing the powder layer to multiple laser scans
at preselected energies designed to maintain the fusible material
at or just above its melting point results in the molten material
flowing together in discreet incremental steps that improves
fusion, strength, and density of the parts while preserving precise
dimensions and in the absence of unwanted growth. Multiple and
successively different energy levels can be delivered by the laser
108 either via software or by overlaying .STL files, each having a
different laser energy level setting.
[0034] Typically, the first scan of the layer is fully completed
before the second scan begins and sufficient dwell time is provided
to allow the heated layer to flow and cool slightly so as to
dissipate heat to the part cake, but still maintain the fused area
as a liquid. It should be recognized that dual beam lasers can also
be used in the practice of the invention in which there is no
cooling time between scans, so long as over-melting and consequent
growth do not occur. It should also be recognized that, depending
upon the energy of the laser or the size of the area scanned, each
raster or vector scan can be rescanned immediately, although not
necessarily with equivalent results. If desired, the laser beam
diameter and spacing can be varied between subsequent scans.
[0035] FIGS. 3A, 3B, and 3C illustrate a comparison of the
conventional scanning method (FIG. 3A) to that of the invention
(FIGS. 3B and 3C). As shown in FIG. 3A, a conventional scanning
method typically includes scanning the powder with the laser at a
high energy level in a single scan. In an attempt to improve the
density of the part, the powder is typically heated with the laser
to a temperature that is significantly higher than the melting
point of the powder. This higher energy laser scan may cause a
larger percentage of the powder particles to have a low viscosity
and remain in a low viscosity state for an extended period of time.
As a result, the part may experience undesirable distortion from
the desired dimensions.
[0036] In contrast, FIG. 3B illustrates that scanning with three
successive equal scans at low energies that are separated in time
results in the powder being heated slightly above its melting
point. The amount of energy directed at the powder should be high
enough to allow the viscous material to flow in a controlled manner
and cool before any undesirable distortion of the part can
occur.
[0037] FIG. 3C shows an initial laser pulse heating and melting the
fusible powder with subsequent pulses of reduced power to continue
to heat the already melted material to maintain the material at or
slightly above the melting point.
[0038] According to one aspect of the invention, the method
provides a means whereby distortion of the part may be reduced.
Typically, the resulting dimensional accuracy of the final part
should have minimal growth in comparison to the nominal value for
the part. The term "nominal value" refers to the expected value for
the part that is input or calculated by the CAD/CAM software.
Typically, the closer the mean dimensional value is to the nominal
value, the more dimensionally accurate is the resulting part.
[0039] In another aspect of the invention, scanning the powder with
multiple laser scans also allows the density of the part to be
increased up to a maximum limit. While not a precise correlation,
it is generally true that an increase in density reflects an
increase in strength, so long as the dimensions are acceptable.
Typically, increasing the density will improve the part's
mechanical stability including its tensile strength, percent yield
at break, and tensile modulus. It has been found that even small
changes in the part's density can significantly affect the
resulting mechanical stability of the part.
[0040] Preferably, the density of the resulting part is close or
equivalent to the ultimate density that is possible for the
material from which the part is composed. The term "ultimate
density" corresponds in meaning to describing a part as fully
dense, which is that density of a mass of the material melted in a
vacuum. Injection molded parts are typically fully dense or nearly
fully dense. Ultimate density can be defined as characterizing a
part that has no void spaces in its volume, no measurable porosity.
The method of the invention can produce parts having densities that
closely approach ultimate density. The relationship between
ultimate density and the length of time at which a fusible powder
target area is maintained at or slightly above its melting point,
in the absence of growth, is illustrated in FIG. 4.
[0041] While not wishing to be bound by theory, it is believed that
increasing the number of laser scans and reducing the energy of the
laser helps to keep the molten fusible material closer to its
melting point while providing sufficient time and sufficient
lowering of the molten material's viscosity so that fusion of the
molten material may occur at ultimate density. Each scan softens
and melts the outer boundaries of the particles so that the viscous
material flows into the void spaces between the particles in
discrete incremental steps. As a result, with each successive scan
the density of the part can be increased up to a maximum or desired
limit while substantially eliminating any distortion of the
part.
[0042] It should be recognized that the number of scans necessary
to increase the density is dependent upon many factors including,
for example, the powder's physical properties such as its melting
point and viscosity, laser power utilized in the scan, time between
scans, time constraints for producing the part, and the like.
[0043] In another aspect of the invention, the intensity of each
successive scan can be varied to decrease the amount of time that
is needed to produce a part having a desired dimensional accuracy
and density. For instance, the initial scan may have the highest
intensity than the subsequent scans or it may have the highest
intensity with each successive scan having a lower intensity. The
first scan should allow a greater portion of each scanned particle
to reach a higher temperature resulting in a longer cooling period.
As a result, the molten material will have a longer period of time
in which to flow and fuse together. The heat to which the powder is
exposed should be low enough so that growth does not occur. In each
subsequent scan the energy can be reduced to facilitate incremental
controlled fusing of the particles. This process should be more
efficient because it combines higher laser energies in the initial
scan with lower energies in subsequent scans to incrementally
produce the part at a faster rate of speed.
[0044] During the laser sintering process, the part bed 132 in the
powder bed is heated to an equilibrium state that is below its
melting point. When the laser beam is applied to the powder in the
target area, a localized hot zone is created. The temperature rise
of the fused powder can be calculated using its measured heat
capacity, heat of fusion and density. The sintering behavior of
this powder can be modeled using the sintering law described by
Childs et. al. in the 2001 edition of the Rapid Prototyping Journal
at pages 180 through 184 of Volume 7, in an article entitled
"Density Prediction of Crystalline Polymer Sintered Parts at
Various Part cake Temperatures." In the sintering law, the increase
in density with time is related to the sintering progress and is
shown to be a function of both density and temperature:
d.rho./dt=f(.rho.,T)
[0045] As can be seen from the mathematical relationship, the
viscosity decreases and the density generally increases due to void
reduction when the temperature of the material rises. At a constant
temperature, sintering progresses with time, however, the sintering
rate decreases as the density approaches the maximum material
density.
[0046] It is advantageous to raise the powder temperature as hot as
possible to achieve low viscosity. It is also advantageous to
maintain the fused powder at a high temperature for as long as
possible to allow densification to proceed. However, thermal energy
added to the fused or partially fused powder can cause melting of
particles adjacent to the part, which results in undesirable
growth.
[0047] Theoretically then, the ideal sintering case can be modeled
as one where it is desired to add as much heat to the fused part as
possible, yet maintain a heat flux out of the part into the
adjacent powder so that this adjacent powder cannot melt and fuse.
This heat flux will, in general, be limited by the thermal
conductivity of the powder. In other words, adding more power to
the part than the powder can thermally remove will cause melting of
the adjacent particles. Adding power at a rate less than this
theoretical limit will prevent heat buildup and limit powder
melting. In the absence of some other active cooling mechanism,
this practically limits the amount of power that can be added to
the part per unit time.
[0048] The multiple scanning techniques of the invention improve
the sintering/densification rate of powder by keeping the part
temperature high, but not so high as to result growth, while
simultaneously providing time for sintering via viscous flow within
the target area.
EXAMPLES
[0049] The following Examples are for illustrative purposes only
and should not be considered limiting in any way.
[0050] In the following examples, the samples were prepared by
sintering a layer of DuraForm.RTM. nylon powder with from 1 to 3
scans at varying laser intensities. A 100 watt high speed
Vanguard.TM. V207 laser sintering system available from 3D Systems,
Inc. was used to form the samples.
[0051] For convenience, the examples in Table 1 below illustrate
multiple scans of equal low power. In some cases, the examples show
relative tensile elongation improvements of 100% or better compared
to a conventional single scan technique (column for 1 scan in Table
1) and improvements of approximately 30% in elongation compared to
elongations of approximately 10% for the conventional
technique.
[0052] It should be recognized that multiple scans at too high a
power will result in growth, as shown above in Table 1 for the
example of 10 scans at 7 watts.
[0053] Multiple scanning is desirably accomplished at varying laser
powers. It is sometimes preferable to use a relatively high laser
pulse on the first scan compared to subsequent scans since the
freshly deposited powder is relatively cold compared to previously
melted material in the same layer. Successive subsequent scans can
have successively lower energy levels. In this way, additional heat
can be added to the powder to quickly raise its internal energy
close to the temperature at which the sintering rate becomes
significant. Subsequent scans are then used to maintain the fused
powder mass at this higher temperature. The use of a higher power
initial laser scan normally should result in faster part production
while maintaining the heat flow needed to minimize growth. However,
it remains important not to use too high a power on the first scan
of the first layer of a part because excessive laser power will
bleed through the layer and cause growth on the powder below the
desired plane of the part. Subsequent layers should not suffer from
this problem since they already have fused material beneath
them.
[0054] Strength improvements of 100% or better are shown by using
multiple full power scans separated by delay times that allow for
excess thermal energy to dissipate. In this example, shown in Table
2 below, the target area is scanned three times by the laser,
followed by long multiple delays of sacrificial parts that serve to
act as a time delay mechanism. The desired parts are then rescanned
again within the same layer, again followed by the delay parts.
TABLE-US-00001 TABLE 2 Time to scan Time to scan Time to scan
sacrificial parts Layer tensile bars beam coupon (delay time) 1
(bottom) 22 5 3 parts * 10 scans = 90 sec 2 22 5 1.66 parts * 10
scans = 45 sec 3 (top) 22 5 1 parts * 10 scans = 30 sec Layer
Tensile % E Modulus 1 (bottom) 7100 15 250 2 7100 21 240 3 (top)
6800 19 230 DFPA standard 4800 8 210
[0055] While not wishing to be bound by theory, the heat transfer
out of the freshly fused powder is considered to be a function of
its local environment, whether the freshly fused powder sits in a
block of fused powder or exists as a thin line of fused powder in a
cake of unfused powder. This local environment affects the heat
flow by allowing or restricting access to the part cake. In the
case of a fused block, the interior of the block transfers heat
across the volume of the part to the part cake. This pathway may be
long and complex, reducing the heat transfer rate and increasing
the possibility of unwanted growth. On the other hand, a thin line
of fused powder has a large amount of surface area in proximity to
the part bed and heat transfer can occur quite easily.
[0056] Tables 3 through 6 below illustrate additional examples of
multiple scanning techniques of the invention and the impact of
laser power, the number of scans, and the delay time between scans
on the quality of the parts produced. In these examples, a delay or
dwell time occurred in those examples having multiple scans between
the end of one scan and the beginning of another. The dwell time
allows the powder to cool and heat to dissipate, but generally the
dwell time was not so long that the powder cooled much below its
melting point.
[0057] Table 4 shows the weights obtained for the examples of Table
3 and Table 6 shows the weights obtained for the examples of Table
5. Example 4 in Table 3 shows that too high a laser power with
inadequate delay between scans resulted in unwanted part growth.
Comparing Examples 8, 9 and 2 to 1 and 7, respectively, it is seen
that while the conventional single scan (Examples 1 and 7) and the
multiple scan technique of the invention (Examples 8, 9, and 2)
each resulted in parts with acceptable or at least not significant
growth, the multiple scan technique of the invention resulted in
significant increases in weight, which generally correlates with
density and improved mechanical properties. Those examples showing
increased weight and not having acceptable growth do not accurately
reflect density since the shape is typically irregular.
Improvements in mechanical properties are borne out in the data
obtained for tensile strength, % elongation, and tensile modulus
for those examples showing multiple scans and acceptable growth.
The poor quality of Examples 3 thorough 6 is attributed to use of
too high a laser power for too many scans with inadequate heat
dissipation between scans.
[0058] Comparing Examples 1, 4, and 7 of the conventional single
scan technique to Examples 3 and 8 of Tables 5 and 6 shows
improvements in density and mechanical properties by the practice
of the invention. Examples 2, 5, 6, and 9 are consistent with
Tables 3 and 4 in that multiple scans at too intense a laser power
can result in unwanted growth. TABLE-US-00002 TABLE 3 Tensile %
Tensile Number of Laser Power Width Strength Elongation Modulus
Thickness Run No. Quality* MLT** Scans in Watts (in) (psi).+-. at
break.+-. (kpsi).+-. (in) 1 5 15 1 50 0.514 3776 5.2 194 0.124 2 4
15 2 60 0.529 6088 6.9 273 0.132 3 0 15 3 70 0.587 5022 7.4 235
0.199 4 0 30 3 60 0.542 5279 7.8 256 0.178 5 0 30 2 70 0.528 6057
6.9 281 0.141 6 0 30 3 50 0.533 6091 8.8 283 0.145 7 5 60 1 70
0.517 5438 4.9 262 0.129 8 5 60 2 50 0.517 6397 7.9 278 0.126 9 4
60 3 60 0.534 6642 8.4 280 0.131 *Quality is a subjective
determination based on the appearance of growth in the part, valued
from 0 to 5, 5 indicating the absence of undesirable growth. **MLT
designates "minimum laser time" and refers to the number of seconds
between the start of one laser scan and the next, including any
dwell in time in between. .+-.Values determined according to the
standard set forth in ASTM D-638.
[0059] TABLE-US-00003 TABLE 4 Part Weights in Grams Run Run Run Run
Run Run Run Run Run 1 2 3 4 5 6 7 8 9 7.4 9.4 15.0 12.5 9.9 10.6
8.4 8.7 10.2 7.5 9.2 14.1 12.4 10.2 10.6 8.4 8.9 10.3 7.5 9.3 14.7
11.7 9.9 10.6 8.4 8.8 10.1 7.7 9.4 14.9 12.3 9.8 10.3 8.4 8.8 10.1
7.5 9.3 14.7 12.1 10.0 10.3 8.4 8.8 10.2
[0060] TABLE-US-00004 TABLE 5 Tensile % Tensile Number of Laser
Power Width Strength Elongation Modulus Thickness Run No. Quality*
MLT** Scans in Watts (in) (psi).+-. at break.+-. (kpsi).+-. (in) 1
+5 30 1 14 0.508 6004 4.1 280 0.122 2 +1 30 2 19 0.523 6300 5.6 285
0.145 3 -5 30 3 25 0.590 5695 6.0 286 0.200 4 +5 60 1 19 0.511 6530
4.7 301 0.128 5 0 60 2 25 0.540 5540 5.3 261 0.173 6 0 60 3 14
0.523 6338 7.3 285 0.152 7 +5 90 1 25 0.513 6387 5.8 298 0.126 8 +5
90 2 14 0.512 6762 8.0 293 0.134 9 0 90 3 19 0.544 5560 10.0 227
0.176 *Quality is a subjective determination based on the
appearance of growth in the part, valued from 0 to 5, 5 indicating
the absence of undesirable growth. **MLT designates "minimum laser
time" and refers to the number of seconds between the start of one
laser scan and the next, including any dwell in time in between.
.+-.Values determined according to the standard set forth in ASTM
D-638.
[0061] TABLE-US-00005 TABLE 6 Part Weights in Grams Run Run Run Run
Run Run Run Run Run 1 2 3 4 5 6 7 8 9 8.1 9.9 15.2 8.5 11.7 10.3
8.4 9.1 11.8 7.9 9.6 15.2 8.4 11.3 10.0 8.4 9.1 11.4 8.3 9.6 14.9
8.5 11.2 9.8 8.5 8.9 11.3 8.0 9.8 15.6 8.4 11.6 10.1 8.6 8.9 11.6
7.9 10.3 15.7 8.4 12.0 10.3 8.5 8.9 11.8
[0062] Many modifications and other embodiments of the invention
set forth herein will come to mind to one skilled in the art to
which the invention pertains having the benefit of the teachings
presented in the foregoing descriptions and the associated
drawings. Therefore, it is to be understood that the invention is
not to be limited to the specific embodiments disclosed and that
modifications and other embodiments are intended to be included
within the scope of the appended claims. Although specific terms
are employed herein, they are used in a generic and descriptive
sense only and not for purposes of limitation. All patents and
patent applications referenced herein are hereby specifically
incorporated by reference in pertinent part.
* * * * *